The invention relates generally to the field of ceramic materials and processes for making ceramic materials. More specifically, the invention relates to silicon carbide bodies having particles or inclusions which are dispersed within the bodies.
Silicon carbide is useful in a wide variety of applications due to its tribological characteristics and its outstanding thermal, chemical and mechanical properties. Such applications include, for example, mechanical seals, valve lifters, and other applications where a part is frictionally engaged with another material. For example, in many mechanical seal applications, the seal interfaces are subjected to both a large compressive stress normal to the seal surface and to high rotational speeds or sliding velocities. Such conditions are typically represented by the parameter PV which represents the product of the compressive stress and the sliding velocity.
When such a mechanical seal is used in a pump or agitator, the mechanical seal should provide adequate sealing of the working fluid. Conveniently, the working fluid may also serve to lubricate and cool the seal interface. If sufficient lubrication and cooling is not provided, excessive wear or catastrophic failure of the mechanical seal may result. For example, if insufficient fluid is provided at the seal interface during operation, the lubricant can vaporize due to the heat produced and cause catastrophic failure.
Hence, when a silicon carbide body is used in a seal or other bearing face which runs against the face of another material, the seal or bearing face should be exposed to a lubricating and cooling fluid (or used in fluid applications) so that a film may be produced between the sliding surfaces to lubricate and cool the surfaces, thereby reducing friction, wear, and temperature as well as catastrophic failure potential. Further, proper lubrication will tend to minimize power consumption.
To facilitate proper lubrication, a variety of silicon carbide materials have been proposed. These include both reaction bonded silicon carbide materials and sintered silicon carbide materials with special modifications to the standard product. The reaction bonded silicon carbide materials are produced by placing a carbon containing preform in contact with molten silicon. As examples of such processes, U.S. Pat. Nos. 6,398,991, 4,536,449 and 4,120,731 describe reaction bonded silicon carbide bodies having secondary lubricating particles dispersed therein. The complete disclosures of these patents are herein incorporated by reference. Sintered silicon carbide materials are produced by combining a dry lubricant agglomerate and a silicon carbide matrix formulation, drying and compacting the mixture into a green body, and heating or sintering. An example of sintered silicon carbide is found in U.S. Pat. No. 5,656,563, the disclosure of which is herein incorporated by reference.
The above processes have met with limited success for a variety of reasons. For example, the processes used to produce such materials are often complex and can therefore be relatively expensive. For instance, as recognized in U.S. Pat. No. 5,656,563, it is difficult to incorporate large amounts of graphite into a ceramic matrix without causing cracks to occur in the microstructure or without increasing the material's porosity. The use of graphite or other dry lubricants with silicon carbide bodies tends to result in unwanted lamination of the bodies. Further, crushed dry lubricants typically produce irregular-shaped inclusions in the silicon carbide body. The irregular shaped particles, and their associated sharp edges, produce stress risers within the ceramic body, resulting in a weaker ceramic body and decreased tribological performance.
Hence, it would be desirable to provide silicon carbide materials having improved strength and stability, while maintaining the lubricity of the component. It would further be desirable to provide exemplary processes for making such materials. Such processes should be relatively simple so that the overall cost of the material may be reduced. Such a silicon carbide material should also be useful in applications having a high PV value or temporary dry running applications while reducing the chances of catastrophic failure, excessive wear, and power consumption.